1. Field of the Invention
The invention relates to a low-noise amplifier, a circuit for controlling the same, and a communication terminal including the same, and more particularly to a low-noise amplifier (LNA) to be used in a receiver circuit in a mobile communication terminal and a small-sized satellite communication terminal, a circuit for controlling the same, and a communication terminal including the same.
2. Description of the Related Art
In general, a low-noise amplifier is comprised of a high electron mobility transistor (HEMT) or a metal semiconductor field effect transistor (MES FET). A low-noise amplifier is formed quite small in order to have low noise with a high frequency, in particular, a frequency in a micro-wave band. As a result, a low-noise amplifier is generally weak in response to an excessive input.
Thus, many attempts have been made to protect a low-noise amplifier from an excessive input by incorporating a protection circuit into an input section, as follows.
Japanese Unexamined Patent Publication No. 62-66705 has suggested one of such attempts.
Japanese Unexamined Patent Publication No. 63-149909 has suggested a low-noise amplifier circuit including a low-noise amplifier comprised of a field effect transistor, a level detector for detecting a level at which the low-noise amplifier receives a signal, and a gate bias controller for controlling a bias voltage to be applied to a gate of the field effect transistor in accordance with a level detected by the level detector. The gate bias controller increases a drain current in the field effect transistor in accordance with an increase in the detected level, and provides a bias voltage at an optimal point of a noise index when a low level is received, and provides a bias voltage at a maximum saturation output point when a high level is received.
Japanese Unexamined Patent Publication No. 3-209905 has suggested an amplifier including an input terminal, an input matching circuit electrically connected to the input terminal, an amplifying device electrically connected to the input matching circuit, an output matching circuit electrically connected to the amplifying device, and an output terminal electrically connected to the output matching circuit. The input matching circuit is comprised of a micro-strip line as a transmission line, and a plurality of stubs formed in a micro-strip pattern and electrically connected in parallel with the transmission line. At least one of the stubs is a capacitive stub having an about 1/4 wavelength at an unnecessary signal frequency. The input matching circuit acts as a noise matching circuit for a frequency band of a signal to be amplified, and acts as a filter for attenuating a frequency of an unnecessary signal.
Japanese Unexamined Patent Publication No. 4-243307 has suggested a low-noise amplifier including a first field effect transistor for accomplishing high-frequency, low-noise amplification, a second field effect transistor for doing the same, a first 1/4 wavelength micro-strip lines electrically connected between an output terminal of the first field effect transistor and a common output terminal, and a second 1/4 wavelength micro-strip lines electrically connected between an output terminal of the second field effect transistor and a common output terminal.
Japanese Unexamined Patent Publication No. 5-267957 has suggested a low-noise converter including NPN bipolar transistor formed in an auto-bias circuit and having an emitter to which a current feedback resistor is electrically connected. The current feedback resistor is comprised of a temperature sensitive resistor having a negative temperature coefficient. The current feedback resistor decreases a drain current in a field effect transistor as a low-noise amplifier in accordance with reduction in a temperature. As a result, an increase in gain is prevented at low temperature, ensuring prevention of oscillation.
Japanese Unexamined Patent Publication No. 8-307305 has suggested a circuit for transmitting and receiving signals in a transceiver which circuit enhances isolation between signal receipt and signal transmission in a radio transceiver.
However, incorporation of a protection circuit into a low-noise amplifier is accompanied with a circuit loss, resulting in degradation in a noise temperature, and hence, destruction of an advantageous characteristic of a low-noise amplifier.
Even if such an attempt is made that a low-noise amplifier is less influenced in a selected frequency or in a frequency band at which a signal is received, it would be impossible to completely eliminate a circuit loss. In addition, such an attempt becomes meaningless, if an excessive input is applied to a selected frequency or a frequency band at which a signal is received.
In a mobile communication terminal, a power source for providing power to a receiver low-noise amplifier is generally turned off in order to reduce current consumption, while the receiver low-noise amplifier is not in operation, for instance, while a signal is being transmitted or while the mobile communication terminal is waiting for signals. The mobile communication terminal is weaker to an excessive input while a signal is being transmitted or while the mobile communication terminal is waiting for signals than while the mobile communication terminal is in operation.